Question: In chemistry class, the girl to boy ratio is $4$ to $2$. If there are a total of $18$ students, how many girls are there?
A ratio of $4$ girls to $2$ boys means that a set of $6$ students will have $4$ girls and $2$ boys. A class of $18$ students has $3$ sets of $6$ students. Because we know that there are $4$ girls in each set of $6$ students, the class must have $3$ groups of $4$ girls each. There is a total of $12$ girls in chemistry class.